1. Field of the Invention
The present invention generally relates to an optical beam scanner, an image forming device, and an optical beam scanning method, and more specifically, to an optical beam scanner, an image forming device, and an optical beam scanning method realizing forming a high quality image by using optical beam scanning according to a rotary polygon mirror.
2. Description of the Related Art
Conventionally, as an optical beam scanner used in an image forming device such as a copier, a facsimile, a printer, a printing press, and the like, Japanese Laid-Open Patent Application Publication No. S64-73369 discloses an optical beam scanner which is configured to rotate plural rotary polygon mirrors which deflect and scan an optical beam onto an image supporter and detect rotating positions of the rotary polygon mirrors so that the rotary polygon mirrors are rotated at constant speed.
In the above described optical beam scanner, the plural rotary polygon mirrors are driven to rotate at constant speed by respective driving units in accordance with rotation reference signals corresponding to the rotary polygon mirrors and output signals of the detected rotating positions.
Further, in an image forming device including the above described optical beam scanner, independent latent images are formed on the corresponding image supporters according to an optical beam which is deflected and scanned by the rotary polygon mirrors. Then, the latent images developed to be pixel images are superposed and transferred onto a recording medium.
In order to superpose the pixel images at respective correct positions on the recording medium, positions for starting image forming in the main scanning direction and the sub scanning direction of the pixel images on the image supporter must be adjusted correctly.
As for the main scanning direction, the optical beam is detected at a predetermined position on a scanning path, and timing for writing each scanning line of the pixel images is adjusted in accordance with the detection result. Accordingly, it is possible to prevent an image shift in the main scanning direction. On the other hand, as for the sub scanning direction, where plural image supporters are provided corresponding to the plural rotary polygon mirrors, an interval between the image supporters is set equal to an integer multiple of a scanning pitch. The timing for starting writing the pixel images is adjusted at a unit of period necessary for one optical beam scanning. Accordingly, it is possible to prevent an image shift greater than one scanning pitch in the sub scanning direction over the entire image.
As for a technology disclosed in Japanese Laid-Open Patent Application Publication No. S64-73369, a PLL (Phase Locked Loop) control unit starts controlling rotational speed of a rotary polyhedron (rotary polygon mirror) in accordance with a reference frequency signal generated by the reference frequency signal generation unit. Further, synchronizing sensors generate horizontal synchronizing signals by receiving a laser beam deflected by the rotary polyhedron rotating at constant speed. Then, a timing measurement unit synchronizes the horizontal synchronizing signal output from one of the synchronizing sensors, and measures a timing difference of the horizontal synchronizing signals output from the other synchronizing sensors in accordance with the reference frequency signal. A phase control unit adjusts a phase of the reference frequency signal supplied to the PLL control unit in accordance with the measured timing difference of the output horizontal synchronizing signals.
With this structure, it is possible to adjust a phase shift minutely within a pixel distance, and a shift of the top scanning line of the laser beam on the photosensitive drums (image supporters) may be set minimum.
In the case where both sides (the first surface (a surface) and the second surface (the other surface)) of the object to be printed are printed by utilizing the above described conventional image forming device, the pixel images (toner) transferred onto the object to be printed are heated and fused, and thus moisture in the sheet is evaporated so that the object size is reduced. In this case, for example, when the object having two surfaces such as a sheet is printed, i.e., both a printing region on the first surface and a printing region on the second surface of the object are printed, the size of the object to be printed is reduced upon heating and fusing the image transferred onto the first surface to be printed. Accordingly, the rotational speed of the rotary polygon mirror should be switched twice, i.e., after completing drawing (printing) the first surface until starting drawing (printing) the second surface, and after drawing the second surface until starting drawing a first surface of another sheet.
However, even if the rotational speed of the rotary polygon mirror is switched according to the rotation reference signal, the inertia (inertial energy) of the polygon motor is still high. It is difficult to immediately change the rotational speed in accordance with the rotation reference signal. Further, a period necessary to repeat acceleration and deceleration until the rotation is stabilized depends on the respective polygon motors.
Moreover, the rotational speed of the rotary polygon mirror is switched in accordance with an order of printing. Hence, there is a period when the rotational speed of the rotary polygon mirror to be a reference is different from the rotational speed of the other rotary polygon mirrors. Accordingly, there is a problem in that a position to print in the sub scanning direction of the other rotary polygon mirrors cannot be obtained in accordance with the horizontal synchronizing signal corresponding to the reference rotary polygon mirror.